1. Field of the Invention
The present invention relates to filters and duplexers, and more particularly, to a filter using an acoustic wave device and a duplexer using the same.
2. Description of the Related Art
Recently, a bandpass filter using an acoustic wave device has been used in high-frequency radio equipment such as a cellular phone. Recent advance of performance of cellular phones requires reduction of the absolute value of a temperature coefficient of frequency (frequently abbreviated as TCF). TCF is a rate of change of the frequency response to a variation in the environment temperature. In the resonators, a change of the resonance frequency to a variation in the environment temperature equal to 1° C. is expressed in the unit of ppm/° C. TCF almost depends on the temperature coefficient of velocity of SAW propagated on the surface of the piezoelectric substrate. The TCF of the SAW device is as bad as −80˜−40 ppm/° C. for a piezoelectric substrate of lithium niobate (LiNbO3) or lithium tantalate (LiTaO3), and is thus required to be improved.
Japanese Patent Application Publication No. 2003-209458 discloses an acoustic wave device in which comb electrodes on a substrate of lithium niobate are coated with a silicon oxide film. The acoustic wave is propagated in not only the lithium niobate substrate but also in the silicon oxide film. The temperature coefficient of the propagation velocity of the acoustic wave in the silicon oxide film has a sign opposite to that of the temperature coefficient of the propagation velocity of the acoustic wave in the lithium niobate substrate. It is thus possible to keep the total propagation velocity of the propagation velocity constant irrespective of temperature by optimizing the thickness of the silicon oxide film. That is, TCF can be reduced by optimally selecting the thickness of the silicon oxide film.
FIG. 1 is a graph of a bandpass characteristic of a bandpass filter. By using the technique disclosed in the above-mentioned application, The TCF of a shoulder a on the high-frequency side of the pass band can be reduced to approximately zero. However, a shoulder b on the low-frequency side of the pass band is not improved well. For example, when the temperature raises, the shoulder b shifts to b′.